move acts_as_list to gems (empty vendor/plugins)

fixes CNVS-12056

test plan:
  * ordering/reordering of modules/module items should still work
  * ordering/reordering of discussion topics should still work

Change-Id: I599b559933a4a89c9329d631d71f83343c6f24da
Reviewed-on: https://gerrit.instructure.com/32516
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>
Reviewed-by: Cameron Sutter <csutter@instructure.com>
QA-Review: Matt Fairbourn <mfairbourn@instructure.com>